string_bytes: Make base64 encode/decode reusable

Node already has support for base64 encoding and decoding that is not
visible outside the string_bytes.cc file. Our work on providing a
support for the Chrome inspector protocol
(https://github.com/nodejs/node/pull/6792) requires base64 encoding
support. Rather then introducing a second copy of the base64 encoder,
we suggest moving this code into a separate header.

PR-URL: https://github.com/nodejs/node/pull/6910


Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: indutny - Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: ofrobots - Ali Ijaz Sheikh <ofrobots@google.com>